The load requirements of frozen beverage product dispensers are highly variable, since customer demand for beverages can vary from no drinks dispensed per minute to as many as 3 or 4 or more drinks served per minute. This volatile variation in customer demand results in a very broad range of cooling load requirements for a typical frozen product dispenser, such for example as is shown by the chart of FIG. 2. As can be seen, depending upon ambient temperature and during periods when no product is being drawn, the maintenance cooling load of a frozen product dispenser can be as low as 1500 Btu/hr. At the other extreme and during periods of high drink draw rates, for example when delivering drinks at the rate of 4×16 oz drinks per minute, cooling load requirements of a frozen product dispenser can be on the order of 18,000 Btu/hr. This represents a 12:1 turndown ratio, which from an energy standpoint conventional refrigeration systems are not able to efficiently accommodate.
As is known, conventional frozen product dispensers utilize a compressor that delivers refrigerant through a condenser to one or more expansion valves, each of controls delivery of refrigerant to an associated evaporator cooling coil that is thermally coupled to an associated product barrel in order to chill the product barrel to freeze product in the barrel. To accommodate various cooling load requirements of the product barrels, the expansion valves advantageously are variably controlled, such that as load requirements for an evaporator coil change due to changing customer demands, the expansion valve supplying refrigerant to the evaporator changes to a more appropriate refrigerant flow metering position. The idea is to adjust the expansion valve so as to match the cooling capability of the evaporator, based upon refrigerant flow to the evaporator, more closely to the dynamically changing cooling load requirements of the product barrel that is being chilled by the evaporator. However, fixed speed compressors of a type normally used for frozen product dispensers are not readily able to accommodate changes in cooling load requirements and are best suited to provide refrigerant flow at a certain rate, despite changes in the cooling load. Refrigeration system balance therefore becomes disturbed as the expansion valves are adjusted to meet changing cooling load requirements, resulting in saturated evaporator temperature dropping as cooling load requirements decrease, rising as cooling load requirements increase, and generally poor control over the temperature of the evaporator. In addition, when cooling load requirements decrease, cooling of product in the barrel is quickly satisfied and the compressor must be cycled off. In consequence, where the compressor is not matched with the cooling load, during periods of low product demand the compressor will cycle on/off excessively and the system will operate less efficiently and use more energy than would otherwise be required.
Various known design strategies can be employed to better balance the refrigeration system of frozen product dispensers. One such strategy contemplates using hot-gas bypass to create a “false load” for the compressor, but often there are reasons involving cost and/or complexity of design that rule out using a hot-gas bypass technique. Another strategy involves modulation of compressor pumping capacity to better match compressor cooling capacity to the actual cooling load requirements of the product barrels, but while compressor modulation can be provided by mechanical means in larger capacity compressors, doing so can be cost prohibitive when using smaller compressors of a size as are usually employed in frozen product dispensers.